Failures of the Ethernet Local Area Network (LAN) ports have been attributed to use of inappropriate Surge Protective Devices (SPDs), lack of insulation coordination and inappropriate wiring, which caused the failure of transformers, associated wiring, components and connectors. This guide discusses isolation transformer parameters and how they influence the equipment common-mode and differential-mode surge performance. A copy of [ITU-T K.95] is necessary to fully understand this document.